To enable charging of a battery in a vehicle from the outside, in addition to a charge cable, a communication line used to transmit a control pilot signal (CPLT signal) connects the vehicle and an external power supply device. More specifically, an upper limit value of current that can be supplied from the external power supply device is expressed by a time ratio of the time of logic H to a cycle of logic H and logic L of the CPLT signal, and the upper limit value is notified to the vehicle. This allows the vehicle to perform charge control so that the charge current value is less than or equal to the upper limit value.
Further, a high-frequency signal having a higher frequency than the CPRT signal is superposed on the CPLT signal to perform inband communication that transmits and receives more information than that contained in the CPLT signal (Patent Document 1). In addition, the inband communication is performed with the time ratio of the CPLT signal fixed at 5%. More specifically, when the time ratio of the CPLT signal is 5%, the time ratio is not a value that expresses the upper limit value of current that can be supplied. Rather, the time ratio is a value set in particular to perform the inband communication.
Patent Document 1: International Publication No. 2013/129038